Physicist Albert Abraham A. Michelson, front left, and Albert Einstein, center, at the California Institute of Technology, in Pasadena, on Jan. 7, 1931. The precise light speed measurements taken by Michelson in the San Gabriel Mountains between 1924 and 1927, were critical to verifying Einstein’s Theory of Relativity.

In the summer of 1926, an experimental beam of light pierced the night sky between two precisely measured peaks in the San Gabriel Mountains, and science history was made. Virtually unseen by the major population centers below, the beam was the key element of Dr. Albert A. Michelson’s revolutionary experiments to accurately measure the speed of light.

The light speed experiments performed in the mountains of Southern California between 1924 and 1927, were the culmination of Michelson’s remarkable 40-year career in the study of physics.

Albert Abraham Michelson was born in Strenlo, Prussia, on Dec. 19, 1852. His family immigrated to the United States in 1854, and they eventually settled in San Francisco. Michelson joined the Naval Academy at Annapolis, where he began his studies in physics. His passion was studying the velocity of light, and he went on to become a distinguished professor of physics. In 1907, Albert Michelson became the first American to be awarded the Nobel Prize in Physics.

Michelson spent much of his career developing new methods to measure the speed of light with increased accuracy. In the early 1920’s, he devised a plan to bounce a beam of light between two distant mountain peaks, and measure the speed of light with the greatest accuracy to date.

The measuring stations were set up on Mount Wilson, above Pasadena, and on “Lookout Mountain,” a nondescript peak located 3.2 miles southwest of Mount San Antonio (aka Mount Baldy) summit, above Upland.

Mount Wilson had been used as an observatory site as early as 1889, and the Mount Wilson Solar Observatory was built on the summit in 1904. Numerous astronomical facilities were built on the 5,715 foot peak, and Mount Wilson became a gathering place for famous astronomers and scientists.

Lookout Mountain got its name in 1915, when a fire lookout tower was constructed there. The 6,812 foot peak was originally called “Baldy Lookout,” and it was labeled that way on early forest service maps. The remote tower was only accessible by a steep footpath, and it was connected to the Ontario-Upland telephone exchange so that observers manning the lookout could immediately report fires. Michelson generally referred to the Lookout Mountain site as “San Antonio Mountain,” even though this designation was technically incorrect.

Lookout Mountain and Mount Wilson are separated by approximately 22 miles of extremely rough terrain, and Dr. Michelson’s first challenge was to get a precise measurement of that distance.

The traditional survey methods of the period were not precise enough to support the experiment, so Michelson turned to the U.S. Coast and Geodetic Survey, (U.S.C.G.S.) the finest surveying group in the United States. Michelson’s survey specifications required the measurement between the two mountain stations to be accurate to within 1 part in 500,000. This meant there could only be a variance of about 3 inches in the approximately 22-mile measurement.

The U.S.C.G.S. accepted the challenge, and devised a method to measure the distance by setting up a series of “stations” that could be used to triangulate various distances between the peaks. The survey work led by Lieut. Commander C. L. Garner, began on October 16, 1922, and was completed on March 1, 1923.

Six stations were precisely surveyed and marked with disc-shaped brass medallions, permanently mounted in small concrete monuments. Because of their historic significance, some of these stations still exist today.

The stations at the experiment sites on Mount Wilson and Lookout Mountain are still in existence today, and were originally designated as “Station Michelson,” and “Station Antonio” respectively. Station “Pasadena East Base” is located at the edge of a quarry in Upland, and is still in use today as a survey point. Station “Los Angeles SE Base” was originally established in the late 1880’s, and it has been preserved in a suburban backyard in Garden Grove.

The rugged terrain between some of the stations created some unique and difficult measuring challenges. One survey line passed through an orange grove, and had to be elevated on wooden towers to pass above the citrus trees. Wooden towers were also used to carry the survey lines over steep ridges and deep ravines. In one amusing situation, a house sat directly in the path of the survey, and the accommodating residents allowed the survey line to be run through two fortunately placed windows on the structure.

The historic survey for Michelson’s experiment became known as “The Pasadena Base” survey. Minute precautions were taken at every step to ensure a precise measurement. When the survey was completed, it was regarded as the most precise measurement ever taken by the triangulation method. The final measurement was 35,385.53 meters, or 116,094 feet.

Small structures were built at the Mount Wilson and Lookout Mountain sites to house the light speed test equipment. When the survey and structures were complete, Michelson began running his experiments. His goal was to obtain a speed of light measurement with no more than a 0.001% error.

Michelson devised an ingenious, yet relatively simple method for measuring the speed of light. A beam of light was projected from the Mount Wilson site to a return mirror on Lookout Mountain, and the returning beam hit a rotating mirror assembly at Mount Wilson. The speed of light measurement was obtained by measuring the minute changes to the angle of the light when it was reflected off the rotating mirror.

Michelson’s historic experiments in the San Gabriel Mountains provided the first accurate measurements of the speed of light, and helped pave the way for Albert Einstein to develop his revolutionary Theory of Relativity. In 1931, Einstein praised Michelson’s efforts in a speech at the California Institute of Technology in Pasadena, stating; “Without your work, this theory would today be scarcely more than an interesting speculation.”

In the San Gabriel Mountain experiments, Michelson determined the speed of light to be 299,796 km per second. The modern standard measurement was taken in a vacuum, and places the speed of light at 299,792.458 km per second. Comparing today’s standard light speed figure, Michelson’s measurements were phenomenally accurate, and were only off by 0.0118 percent.

Albert Michelson died in Pasadena, on May 9, 1931, at age 78. In perhaps one of the most ironic pieces of advice ever given, the superintendent of the Annapolis Naval Academy told Michelson; “If you’d give less attention to those scientific things and more to your gunnery, there might come a time when you would know enough to be of some use to your country.”

Mark Landis is a freelance writer for The Sun. He can be reached at Historyinca@yahoo.com

Join the Conversation

We invite you to use our commenting platform to engage in insightful conversations about issues in our community. Although we do not pre-screen comments, we reserve the right at all times to remove any information or materials that are unlawful, threatening, abusive, libelous, defamatory, obscene, vulgar, pornographic, profane, indecent or otherwise objectionable to us, and to disclose any information necessary to satisfy the law, regulation, or government request. We might permanently block any user who abuses these conditions.

If you see comments that you find offensive, please use the “Flag as Inappropriate” feature by hovering over the right side of the post, and pulling down on the arrow that appears. Or, contact our editors by emailing moderator@scng.com.